Willow Slough Dam Reservoir Report

Last Updated: December 12, 2025

Willow Slough Dam, located in Mills County, Iowa, was completed in 1959 and is owned by the state.


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Summary

The dam is primarily used for recreation purposes and has a storage capacity of 1600 acre-feet. With a height of 8 feet and a surface area of 2100 acres, the dam sits on the TR-West Nishnabotna River and is regulated by the Iowa Department of Natural Resources.

The dam, classified as an earth dam with a soil foundation, has a low hazard potential and a moderate risk rating. Although it has not been inspected since 1963, the dam is still considered to be in good condition. In case of emergencies, the dam does not have an Emergency Action Plan (EAP) prepared. Overall, Willow Slough Dam provides recreational opportunities while also serving as a key water resource management structure in the area.

Water resource and climate enthusiasts interested in Willow Slough Dam will find it to be a valuable asset in the conservation and management of water resources in Mills County, Iowa. The dam's presence on the TR-West Nishnabotna River not only offers recreational activities but also plays a crucial role in regulating water flow and storage in the region. Despite its age, the dam continues to function effectively and has a moderate risk rating, highlighting its importance in maintaining water security and safety in the area.

Year Completed

1959

Dam Length

9999

Dam Height

8

River Or Stream

TR-WEST NISHNABOTNA RIVER

Primary Dam Type

Earth

Surface Area

2100

Drainage Area

2.72

Nid Storage

1600

Hazard Potential

Low

Foundations

Soil

Nid Height

8
       
Seasonal Comparison

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Dam Data Reference

Condition Assessment

Satisfactory
No existing or potential dam safety deficiencies are recognized. Acceptable performance is expected under all loading conditions (static, hydrologic, seismic) in accordance with the minimum applicable state or federal regulatory criteria or tolerable risk guidelines.
Fair
No existing dam safety deficiencies are recognized for normal operating conditions. Rare or extreme hydrologic and/or seismic events may result in a dam safety deficiency. Risk may be in the range to take further action. Note: Rare or extreme event is defined by the regulatory agency based on their minimum
Poor A dam safety deficiency is recognized for normal operating conditions which may realistically occur. Remedial action is necessary. POOR may also be used when uncertainties exist as to critical analysis parameters which identify a potential dam safety deficiency. Investigations and studies are necessary.
Unsatisfactory
A dam safety deficiency is recognized that requires immediate or emergency remedial action for problem resolution.
Not Rated
The dam has not been inspected, is not under state or federal jurisdiction, or has been inspected but, for whatever reason, has not been rated.
Not Available
Dams for which the condition assessment is restricted to approved government users.

Hazard Potential Classification

High
Dams assigned the high hazard potential classification are those where failure or mis-operation will probably cause loss of human life.
Significant
Dams assigned the significant hazard potential classification are those dams where failure or mis-operation results in no probable loss of human life but can cause economic loss, environment damage, disruption of lifeline facilities, or impact other concerns. Significant hazard potential classification dams are often located in predominantly rural or agricultural areas but could be in areas with population and significant infrastructure.
Low
Dams assigned the low hazard potential classification are those where failure or mis-operation results in no probable loss of human life and low economic and/or environmental losses. Losses are principally limited to the owner's property.
Undetermined
Dams for which a downstream hazard potential has not been designated or is not provided.
Not Available
Dams for which the downstream hazard potential is restricted to approved government users.